1. Class 13 Introduction to Anonymity CIS 755: Advanced Computer Security Spring 2014 Eugene Vasserman http://www.cis.ksu.edu/~eyv/CIS755_S14/

2. Administrative stuff Quizzes and Exam II – Scheduling – Format

3. Outline Anonymity concepts and background The Dining Cryptographers problem Anonymous email – Chaum mixes – Mixminion Anonymous web browsing – Tor Problems with Tor

4. Anonymity Concepts Privacy – Confidentiality Anonymity/Pseudonymity – Unobservability – Unlinkability

5. Dining Cryptographers Three people toss coins: heads=1, tails=0 Menus hide right-hand coin XOR your coin flip result and left neighbor’s result Report value to everyone Report opposite value to send a single bit If the sum is odd, someone sent a message

6. Dining Cryptographers II Slow Error-prone Needs tamper detection Does not scale Provides unobservability

7. Unobservability k-anonymity (scalable dining cryptographers) – Must be implemented very carefully Link padding – Inefficient – Cover traffic knowledge

8. Unlinkability Sender can’t identify receiver Receiver can’t identify sender Neither knows who the other is – How do we handle authentication? Unobservability implies unlinkability (?)

9. For Bob from Alice For Carol from Alice For David from Alice Onion Encryption

10. Message for Bob Wrapping for Carol Wrapping for Doug Onion Encryption II Bob Alice Wrapping for Edward Edward Doug Carol

11. Chaum Mixes Bob Alice Output in lexographic order

12. Global Adversary Bob Alice

13. Chaum Mix Cascade Bob Alice

14. Anonymous Reply Address for replies: Reply: Mix0 decrypts N,A; sends: Mix decrypting reply does not know destination Mix encrypting reply does not know source

15. Mixminion AB C D E Bob A,B,C,D,E Alice Bob

16. Problems with MixMinon Centralized entities required – Availability failure – Anonymity failure (how?) Malicious nodes: – Control entry and exit – Unlikely

17. Anonymous Email High-latency Low-throughput Provides unlinkability – Have to be careful about authentication No default end-to-end confidentiality (PGP) – Actually, there is for replies Secure against global adversary

18. Anonymous Web Browsing Low-latency Medium-throughput Server does not know client Provides sender unlinkability – Have to be careful about authentication No default end-to-end confidentiality (SSL) NOT secure against global adversary

19. Tor ABC TCP over TCP (UGH!)

20. Problems Tor Does NOT Address Global adversary – Functional vs. actual? Packet counting Packet sampling

21. Anonymous Web Services Web service does not know client Client does not know web service Provides sender and receiver unlinkability Rendezvous

22. Tor Hidden Services ABCDEF

23. Questions? Reading discussion